Interstocks For Tree Size Control

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Title:
Interstocks For Tree Size Control
Physical Description:
Fact sheet
Creator:
Castle, W.S.
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University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
Place of Publication:
Gainesville, Fla.
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Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Melanie Mercer.
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Published
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"Publication date: June 1992."
General Note:
"HS-145"

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University of Florida Institutional Repository
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University of Florida
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All rights reserved by the submitter.
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IR00004626:00001


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FactSheetHS-146 June1992TreeSizeControlandDwarfingRootstocks1 W.S.Castle2Theeconomicsofmoderndaycitrusproduction stronglysuggesttheneedforwiseuseofresources andincreasedyieldsinordertoinsurethe continuanceofacceptableprofits.Theneedto improveyieldisbringingaboutachangeinthe mannerbywhichgroveperformanceisevaluated. Yieldhasgenerallybeenevaluatedonanindividual treebasisinthepast;however,nowitisconsideredin termsoflandarea,reflectingamoreefficientmeans ofutilizingthisbasicresource.Theproductive potentialforagivenlandareathen,isrelateddirectly tofruitbearingfoliage.Thisconcepthasmanifested itselfintheformofhigherdensityplantings.Field trialsofclosely-spacedtreeshavedemonstratedthe capabilityofsuchplantingstoincreaseproductivity. Theadvantagesanddisadvantagesassociatedwith higherdensityplantingshavebeenreported.One consistentobservationhasbeentheneedfor tree-size-controltoameliorateorperhapseliminate thosefactorswhichmaycauseadeclineinyieldsoon aftertheplantingisestablished.Furthermore,and equallyimportant,isthedesiretohavesmallertrees whichmaybemoreeconomicallymanagedand harvested. Treesizecontrolincitruscanbeachievedbyfour generaltechniques-environmental,genetic,viraland mechanical.Anexampleoftheformeristreeson trifoliateorangewhichareoftensmaller-sizedwhen grownindeep,sandysoilsevenwithirrigation. Elsewhere,onbettersoils,theyareslower-growing butreachstandardsize.Variouscombinationsof plantmaterial(geneticapproach)mayalsofavorably affecttreesizeandvigor.Suitablegermplasmhas, unfortunately,beendifficulttofindandonlylimited evaluationofthismaterialhasoccurredsofar.The useofplantpathogenshasbeenexploredespecially citrusexocortisviroid.Itisapromisingtechnique. Theeasiestandmostreliablemethodofsizecontrol ismechanical,i.e.,hedgingandtopping.THECONCEPTOFTREESIZECONTROL VERSESDWARFINGInclassifyingtreesaccordingtosize,dwarftrees arethoseapproximately8ftorlessinheightwhen mature.Treesofthissizehavemanyadvantagesas comparedtolarger,vigorousones.Theycanbe spacedclosertogetherwithoutsufferingfromexcess crowdingortheneedforfrequent,severepruning. Moreover,theygenerallyproducefruitinamore efficientmanner.Smallertreeshaveamorefavorable ratiooffruitbearingfoliagetonon-producingwoody frameworkthanlargertrees. Despiteitsadvantages,smalltreesizeisnotan endinitselfnornecessarilydesirableifpursuedtoits limits.Smalltreesmustbehorticulturallysatisfactory inotheraspects.Also,ifextremelydwarfed,treeswill haveinadequatevolumetoallowmaximum productivity/landareabecauseyieldisfunctionof canopybearingvolumeamongotherfactors. Therefore,smalltreesareusefulforcertainpractical 1.ThisdocumentisFactSheetHS-146,aseriesoftheHorticulturalSciencesDepartment,FloridaCooperativeExtensionService,Institute ofFoodandAgriculturalSciences,UniversityofFlorida.Publicationdate:June1992. 2.W.S.Castle,Professor,Horticulturist,CitrusResearchandEducationCenter,LakeAlfred,Florida,CooperativeExtensionService, InstituteofFoodandAgriculturalSciences,UniversityofFlorida,GainesvilleFL32611. TheInstituteofFoodandAgriculturalSciencesisanequalopportunity/affirmativeactionemployerauthorizedtoprovideresearch,educational informationandotherservicesonlytoindividualsandinstitutionsthatfunctionwithoutregardtorace,color,sex,age,handicap,ornational origin.Forinformationonobtainingotherextensionpublications,contactyourcountyCooperativeExtensionServiceoffice. FloridaCooperativeExtensionService/InstituteofFoodandAgriculturalSciences/UniversityofFlorida/ChristineTaylorStephens,Dean

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TreeSizeControlandDwarfingRootstocks Page2andeconomicreasons,buttheirreducedvolumemay notbeidealoroptimumforbestgroveperformance. Itisimportanttocompare"dwarf"or"dwarfing" and"tree-size-control."Theterm"dwarf"whenused todescribeafruittreeordinarilyevokesanimage withwhichreasonablyspecificdimensionsare associated."Dwarf"thenisapplicableonlytotreesof acertainstaturewithinthefullrangeofpotential sizesresultingfromtheabilitytocontroltreesize. Theuseofrootstocksandvirusesshouldbeviewedas ameansofcontrollingtreesize,notjustdwarfing citrus. Tree-size-controlisaconcept,withwell-defined objectives,thathastwoimportantimplications.First, thisapproachrecognizesandencompassesthe desirabilityandneedforanarrayofplantmaterials, aswellastechniques,tocreateawholespectrumof treesizes,notjustdwarfones;andsecond,itimplies theabilitytoregulatetreesizethroughtheproper combinationofthesematerialsandtechniques.Thus, rootstocksand/orvirusesrepresenttwowaysof controllingtreesize,becausetreesinavarietyofsizes resultfromtheiruse.DEFINITIONOFADWARFINGROOTSTOCKAdwarfingrootstockmaybedefinedassimply onewhichincombinationwithotherplantparts,and independentofviral,environmentalorother influences,resultsinamaturetreenolargerthan8ft (2.5m)inheight.Treesizeincitrus,however, representssomeformofphysiologicalbalance betweeneachrespectivecomponent.Itispossiblethat twosourcesofpropagationmaterialmayexhibita growthpatternincombinationthatisentirely differentfromeitherofthemindividually.Some successintreesizecontrolhasbeenachieved,for example,withcitrusrelativesandspeciesof Citrus eventhoughtherootstockitselfisnotatruedwarf. Adwarfingrootstockmayalsobeatruegenetic dwarfitselfandimpartthischaracteristictoscions buddeduponit.Thistypeofrootstockmayhave greaterpotentialbecauseitisonethatislikelyto producemoreconsistentresults,andbelessaffected byvariationsinenvironmentalandculturalfactors; however,theirslowergrowthandshorterinternodes wouldcomplicatepresentcommercialnursery operationsandprobablyincreasethetimerequiredto produceasaleableplant.IDENTIFYINGDWARFINGROOTSTOCKSGeneticorinherentdwarfismoftenmanifests itselfindistinctivemorphologicalformswhichare easilyidentifiedandobvioustoeventhecasual observer.Typicalvariationsinvolvechangesin internodelength,leafsize,shapeandcolor,growth rate,andfoliagedensityandbranching.These variantsarecommonlyreferredtoas"off-types"and areusuallydiscarded.Evidenceisaccumulatingfrom studieswithapplerootstocksthatdwarfismisrelated togrowthhormones.Verylittleisknownaboutthe hormonephysiologyofcitrusasitcontrolsgrowthbut asamorecompleteunderstandingisachieved,itis reasonabletoassumethatthisinformationwillhelp identifydwarfingrootstocks. Therearenoreportsdescribingatruecitrus dwarfingrootstock.Somestockssuchastrifoliate orange(Poncirustrifoliata Raf.)wereconsidered dwarfinginthepast,butnowarerecognizedassize controllingrootstocksor"rootstockswithdwarfing effect."Treesonthesestocksdonotgenerallyfitthe definitionof"dwarf."Moreover,inthoseinstances wheretree-sizewasreportedlyreduced,dwarfingmay havebeencausedbyotherfactors.DEVELOPINGDWARFINGROOTSTOCKSNewrootstocksbeingevaluatedattheCitrus ResearchandEducationCenter,LakeAlfredsuggest thatfavorabletreesizeisobtainabledespitethe apparentabsenceofgeneticdwarfing.Iftruedwarfs arepreferred,however,oneverypromisingsourcehas beenvirtuallyignored. H.J.Webber,in1932,publishedhisclassicstudy ofvariationsincitrusseedlings.Hestatedthat: "Theseedlingsofvariabletypeswhicharepresent insmallnumbersinalllotsofcitrusseedlings... areheretermedvariants...Theseedlingsofvariant typesareusuallysmallandlackinginvigor,and whenusedasstocksarefoundalmostinvariably toproduceorchardtreesexhibitingsomedegree ofdwarfing." Thesourorange (Citrusaurantium L.)variants (off-types)heselectedrangedfromweak,spindly individualstootherhealthiertypesofdifferentsizes, butallweresmallerthanhis"standard."These

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TreeSizeControlandDwarfingRootstocks Page3variantsalsomaintainedtheircharacteristicseven throughsubsequentpropagationsonroughlemon (Citrusjambhiri Lush.)andsourorangerootstocks. Thisstabilityisagoodindicationthatthevariantsare anexpressionofgeneticchange.Off-typeseedlings couldbearewardingsourceofdwarfingmaterialthat shouldnotbeoverlooked.Furthermore,previous concernsregardingthevegetativepropagationofsuch seedlingsandotherrootstockmaterialshavebeen greatlyreducedconsideringrecentadvancesintissue culturetechniques. Perhapsmoreprogressindwarfingcitrustrees hasbeenmadeinrecentyearsthroughtheworkof W.P.Bitterswiththecitrusrelatives.Representatives fromgenerawithinthesubfamilyAurantioideae excluding Citrus and Poncirus weretriedasinterstocks orrootstocksprimarilywithalemonscion.Results wereencouragingwithspeciesof Citropsis, Eremocitrus, Clymenia and Microcitrus.Tree-sizewas frequentlyreducedto50%and,insomeinstances, 75%ofthestandardcommercialtree.Themechanism ofdwarfingisunknownbutitisapparentthatbud unioncompatibilityisinvolved.Certainrelatives,e.g. Hesperethusa,weresuccessfullypropagatedwithone speciesof Citrus butnotanother. Allpossibilitiesinvolvingthecitrusrelativeshave notbeenexplored.Thevariationmentionedabove precludesanygeneralizationsaboutaparticular genus.AdditionalinvestigationsunderwayinFlorida aredesignedtoevaluatespecific rootstock/interstock/scioncombinations. Presently,advancedtechniquesarebeingusedto mapthegeneticmaterialof Citrus inordertoidentify genesorgroupsofgenesthatcontrolspecifictraits suchasjuiceacidityandtreevigor.Whensufficient informationisaccumulatedthenveryefficientand focussedbreedingwillbepossible.Meanwhile,new rootstocksarebeingcreatedattheCRECbysomatic hybridization(protoplastfusion).Theparentsfor thesenewhybridsincludegermplasmfromselections oftrifoliateorange,e.g.,FlyingDragon,andother speciesthoughttohavepotentialfortreesizecontrol.VIRUSESANDVIROIDSFORTREESIZE CONTROL PotentialrisksDwarfingtreeswithcitrusexocortisviroidis practicedinAustraliaonalimitedbasis;however, commercialimplementationishinderedbyseveral risksanduncertaintiesconcerningexocortisandits interactionwiththehost.Virusinfectedtreesare purportedlyunappealingtomanyindividualsbecause offearsassociatedwithvirusmutationandunwanted transmissiontoadjacenthealthytrees.Althoughthese risksdoexist,theywillprobablyalwaysbepresent andarenotnecessarilycompletelydiscouraging.If exocortisisconsideredahazard,whatrolewould hedgingortoppingplay?Theoretically,theproper combinationoftreeandexocortiswouldnegatethe needfortheseoperations.Ifnearbytreesarenot buddedonasusceptiblerootstock,contaminationis unlikelytohaveaneffect.Furthermore,treeageisa factor.Maturetrees,evenonsusceptiblerootstocks, mayshowno,orslight,effectsfrommechanical inoculation.Itseemsthatexocortisinfluenceontree growthisgreatestwhentreesareinoculatedasyoung plants. Exocortisspreadisalsoinfluencedbythescion variety.Grapefruitandmandarincultivarsappearto berelativelyresistanttomechanicalinoculation. Anotherrisknotcommonlymentionedisthespread ingrovesofthosecultivarswhereclippersareusedto harvestthefruit.Thispossibilityislowhowever, becausegenerallythelargestamountsofviroid particlesarefoundinthebelow-groundpartsofthe citrustree.MutationandVariationCitrusexocortisdoesnotapparentlymutate readilytoformsthataremoredetrimentalthanthe originalone.Millionsofexocortisinfectedtreesexist aroundtheworldwithoutanyapparenthazardsfrom mutation.Inoculatedtreesinfieldtrialshave remainedstableinappearance.Ifanundesired mutationdidoccur,itisnotlikelytobeaproblem evenifdeleteriousbecauseitcouldberestrictedtoa singletree;and,exocortisisnotinsecttransmitted. Variationinthevisualsymptomsofapparently infectedtrees,supportedbyexperimentalwork,has generallybeeninterpretedasevidenceforthe existenceofexocortisstrains.Somestrainsmaycause extremedwarfingandillhealthwhileothersaremuch milderintheireffects.Strainmixturesmayexistin individualplants.Also,ithasbeensuggestedthat anothertransmissiblefactorotherthan,orinaddition to,exocortisisassociatedwiththedwarfing. Recentresearchhasshownthatcitrustreesmay harborahostofviroidswhichhaveyettobefully characterizedregardingtheireffects,ifany,oncitrus.

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TreeSizeControlandDwarfingRootstocks Page4Itisnowknown,e.g.,thatxyloporosisisaviroid disease.Someofthesepreviouslyunknownviroids mayberesponsible,eitheraloneorinvarious combinations,forbehaviorthoughttobecausedby exocortisviroid.